Hydrostatic Circuit Flushing Flow Cancellation

ABSTRACT

Flushing circuits for closed loop hydrostatic circuits enable overriding of the normal function of the flushing circuit during certain machine operational events where flushing can cause undesirable performance issues. The disclosed flushing circuits may prevent flow from leaving the flush valve, may prevent flow from leaving the flushing circuit, may prevent flow from entering the flushing circuit, may prevent the flush valve from shifting from a normally closed position to an open position or may replace the hydro-mechanical control of the flush valve with an electronic control. Cancellation of the flushing function may be commanded by a controller based on fluid temperatures, pressures, turning commands, engine speed, etc.

BACKGROUND

1. Technical Field

This disclosure relates generally to flushing systems for hydrostatic circuits and various means for overriding such flushing systems during certain machine operational events.

2. Description of the Related Art

In the art of hydrostatics, oil or fluid is pumped by mechanical hydrostatic pumps for the purpose of causing a hydrostatic motor to revolve, a hydrostatic cylinder to extend, or for other useful purposes. A common aspect of many tractors, earthmoving machines and the like is a hydrostatic transmission. In its most basic form, a hydrostatic transmission consists of a hydrostatic pump which is normally driven by an internal combustion engine, and provides a source of pressurized fluid flow which causes one of more hydrostatic motors to rotate. The rotation of these one or more hydrostatic motors will cause the machine to travel forward or reverse as commanded by the operator of the machine.

Hydrostatic transmissions typically operate in what is known as a closed loop circuit. In a closed loop circuit, pressurized fluid or oil from a hydrostatic pump flows directly (or through one or more valves) through one line and through a hydrostatic motor before the fluid is returned from the motor through a second line to the pump. The hydrostatic pump and motor are typically of the bidirectional and variable displacement type. This system is known as closed loop circuit because the fluid circulates in a closed path formed by the two lines between the pump and the motor without passing through a fluid reservoir on each pass. This closed loop circuit differs from an open loop circuit where a pump draws the fluid from a fluid reservoir and pumps the fluid through a motor before the fluid is returned to the fluid reservoir. Even in a closed circuit, a small reservoir and a charge pump are needed to collect a small amount of fluid which leaks out of the closed loop and to replace the leaked fluid so that the closed loop remains full of fluid at all times.

When a hydrostatic transmission is operated under heavy loads for an extended period of time, it is possible for the fluid to become heated to an extent which may not be desirable. This heating occurs due to friction and other processes. The fluid may degrade more quickly when maintained at excessive temperatures, thus requiring premature replacement of the fluid. Further, at elevated temperatures, hydrostatic fluid may lose certain lubricating properties including, but not limited to, viscosity. When a hydrostatic fluid loses viscosity, it compromises the fluid's ability to prevent damaging wear to the hydrostatic machinery, such as the pump, motor and valves. In order to remove hot fluid from the closed loop hydrostatic circuit, a “controlled leak” or loop-flushing system is employed to remove fluid from the closed circuit. This fluid is then cooled in a reservoir and returned to the closed circuit through the charge pump.

As shown in FIG. 2 of U.S. Pat. No. 6,430,923, a flushing system 32 may include a flush valve in the form of a spring-centered shuttle spool 34 that is connected to both the high pressure and low pressure fluid paths A, B on the closed loop hydrostatic circuit. The flush valve 34 may be configured to draw fluid from the low pressure line of the two hydrostatic circuit pressure lines A, B of the closed loop system. The flush valve 34 may be connected to a flushing flow regulator valve 36 that may be in communication with a reservoir, a cooler, motor case, etc. The flushing flow regulator valve 36 may control the release of fluid from the loop. The flushing flow regulator valve 36 may also serve to provide a minimal flushing flow of hot fluid from the loop while the charge pump 28 replaces the flushed hot fluid with cool fluid to maintain the fluid in the loop at an appropriate temperature.

Hydrostatic systems include several deficiencies. For example, current loop flushing systems that incorporate a flush valve and a relief valve are not intelligently controlled. Typically, the pressure in the high pressure fluid path dictates when flushing occurs because the system is not intelligently controlled, the loop flushing action occurs whenever the transmission is operational and cannot be overridden. During certain machine operational events, the flushing function can cause performance issues including, but not limited to jerky steering, slow steering response, reduced fluid pressure in the closed loop and sluggish transitions between forward and reverse movements.

Therefore, there is a need for an improved hydrostatic circuit with a flushing function that can be canceled or overridden to avoid the problems noted above.

SUMMARY OF THE DISCLOSURE

In one aspect, a hydrostatic circuit is disclosed that is capable of cancelling its flushing function. The disclosed circuit may include a hydrostatic pump connected to first and second input/output lines. The first and second input/output lines may be connected to a hydrostatic motor to form a loop. The circuit may also include a flush valve, a control valve and a flush outlet. The first and second input/output lines may also be connected to one of the flush valve or the control valve. The flush valve and the control valve may be configured to perform three functions including providing communication between the first input/output line and the flush outlet, providing communication between the second input/output line and the flush outlet and isolating the first and second input/output lines from the flush outlet. The circuit may also include a controller in communication with the control valve for opening the control valve and providing communication between the flush valve and the flush outlet, for closing the control valve and isolating the flush valve from the flush outlet and for reestablishing communication between the flush valve and the flush outlet.

In another aspect, a hydrostatic circuit is disclosed which is also capable of overriding or cancelling the flushing function. The disclosed circuit may include a hydrostatic pump connected to first and second input/output lines. The first and second input/output lines may be connected to a hydrostatic motor to form a loop. The first and second input/output lines may also be connected to a flush valve. The flush valve may include a spool that is moveable between a first position providing communication between the first input/output line and the flush outlet line. The spool may also be moveable to a second position providing communication between the second input/output line and the flush outlet line. Finally, the spool may be moveable to a third position wherein the flush valve isolates the first and second input/output lines from the flush outlet line. The flush outlet line may terminate at a flush outlet. The circuit may further include a controller that is linked to at least one flush override component selected from the group consisting of: a normally open solenoid control valve disposed downstream of the flush valve and upstream of the flush outlet and in communication with the controller and being moveable to a closed position for stopping flow from the flush valve to the flush outlet; a normally open solenoid control valve disposed upstream of the flush valve and in communication with the controller and being moveable to a closed position for stopping flow from the first and second input/output lines to the flush valve; a normally open solenoid control valve disposed upstream of the flush valve and in communication with the controller and being moveable to a closed position for preventing communication between the first and second input/output lines and the flush valve; and a pair of solenoids disposed at opposing ends of the flush valve and in communication with the controller for maintaining the flush valve in its normally closed position. The circuit may further include at least one temperature sensor linked to the controller for communicating a temperature of fluid in the hydrostatic circuit to the controller and a plurality of pressure sensors linked to the controller for communicating pressures in the first and second input/output lines to the controller.

In another aspect, a method for overriding a flushing function of a flush valve of a closed loop hydrostatic circuit is disclosed. The method may including overriding the flushing function in response to at least one operating condition selected from the group consisting of: measuring a temperature of a fluid in the circuit and, if the temperature of the fluid is below a predetermined temperature, sending a signal to stop any flushing flow from the circuit to a flush outlet; measuring a loop pressure of the fluid in the circuit and, if the loop pressure is below a predetermined minimum loop pressure, sending a signal to stop any flushing flow from the circuit to the flush outlet; measuring pressures in first and second input/output lines of the circuit, calculating a difference (ΔP) between the pressures in the first and second input/output lines and, if the ΔP is below a predetermined minimum ΔP, sending a signal to stop any flushing flow from the circuit to the flush outlet; receiving a turn command and sending a signal to stop any flushing flow from the circuit to the flush outlet; receiving a turn command, sending a signal to stop any flushing flow from the circuit to the flush outlet, timing a duration of the turn command and, if the duration of the turn command exceeds a predetermined maximum turning time period, sending a signal to initiate flushing flow from the circuit to the flush outlet and, optionally, receiving a straight steering command and sending a signal to initiate flushing flow from the circuit to the flush outlet in response to receiving the straight steering command.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a closed loop hydrostatic circuit equipped with a prior art flushing circuit, but which can be equipped with any of the disclosed flushing circuits.

FIG. 2 is a schematic illustration of a first disclosed flushing circuit that may be incorporated into the hydrostatic circuit of FIG. 1.

FIG. 3 is a schematic illustration of a third disclosed flushing circuit that may be incorporated into the hydrostatic circuit of FIG. 1.

FIG. 4 is a schematic illustration of a third disclosed flushing circuit that may be incorporated into the hydrostatic circuit of FIG. 1.

FIG. 5 is a schematic illustration of a fourth disclosed flushing circuit that may be incorporated into the hydrostatic circuit of FIG. 1.

FIG. 6 is a schematic illustration of a fifth disclosed flushing circuit that may be incorporated into the hydrostatic circuit of FIG. 1.

FIG. 7 is a schematic illustration of a sixth disclosed flushing circuit that may be incorporated into the hydrostatic circuit of FIG. 1.

FIG. 8 is a flow chart illustrating the cancellation of the flush function when the loop pressure is below a predetermined value.

FIG. 9 is a flow chart illustrating the cancellation of a flushing function if a difference in pressures between the first and second input/output lines is less than a predetermined value and the activation of the flushing function when the tank/loop temperature is above a predetermined value regardless of the low pressure differential between the two input/output lines.

FIG. 10 is a flow chart illustrating the cancellation and activation of the flushing function in response to the fluid temperature.

FIG. 11 is a flow diagram illustrating the cancellation and activation of the flushing function in response to turn commands from the operator or steering mechanism.

FIG. 12 is another flow diagram illustrating the cancellation and activation of flushing in response to various parameters including charge pressure, turn commands, pressure differential between the two input/output lines, and fluid temperature.

DETAILED DESCRIPTION

FIG. 1 illustrates a closed loop hydrostatic circuit 10 for background purposes as FIG. 1 also illustrates typical flushing circuit 11. It will be noted that the hydrostatic circuit 10 may not be the only type of hydrostatic circuit 10 that requires a flushing function. While the flushing circuits 111, 211, 311, 411, 511, 611 illustrated in FIGS. 2-7 may be incorporated into the hydrostatic circuit 10 of FIG. 1, it will be noted that the flushing circuits illustrated in FIGS. 2-7 are not limited to incorporation into the hydrostatic circuit 10 of FIG. 1, but may be incorporated into other hydrostatic circuits as well.

Referring to FIG. 1, the hydrostatic circuit 10 includes a hydrostatic charge pump 12 that is connected to a first input/output line 13 and a second input/output line 14. The lines 13, 14 are described as input/output lines 13, 14 as the hydrostatic charge pump 12 illustrated in FIG. 1 is a variable displacement pump capable of directing flow in either direction, i.e., from the pump 12 through the line 13 to the hydrostatic motor 15 or from the pump 12, through the line 14 to the hydrostatic motor 15. Similarly, the hydrostatic motor 15 is also a two way variable displacement hydrostatic motor that may rotate a shaft or other component 16 in either direction as indicated by the arrows 17. While the circuit 10 is described herein as a closed loop circuit, the hydrostatic fluid or oil flowing through the circuit 10 may need to be filtered or cleaned or the circuit 10 may need to be cooled by removing hot fluid and replacing the removed hot fluid with cooler fluid from a tank or reservoir 18. The reservoir 18 is in communication with a makeup pump 19 which draws fluid from the reservoir 18 and delivers it to the input/output lines 13, 14 via the makeup lines 21, 22 respectively. The makeup lines 21, 22 may be equipped with check valves 23, 24 respectively to prevent backflow from the input/output lines 13, 14 to the makeup pump 19. The makeup lines 21, 22 may also be in communication with a charge relief valve 25 that may be a normally closed, pilot operated pressure relief valve as shown and that may provide communication between the line 26 that is in communication with the makeup lines 21, 22, and the reservoir 18, or an alternative tank or reservoir (not shown).

FIG. 1 also illustrates a conventional flushing circuit 11 that may include a flush valve 27 that may be pilot operated as shown with a shuttle spool 28 that may be moved between three positions. The flush valve 27 is shown in its normally closed position in FIG. 1 wherein flow from the first and second flush lines 31, 32 to the flush outlet line 33 is blocked. In the event the pressure in the first input/output line 13 exceeds a predetermined value, the pressure will be communicated from the line 13, through the line 31 to the first pilot line 34 which will shift the shuttle spool 28 downward in the orientation of FIG. 1 thereby overcoming the bias of the spring or biasing element 35 thereby establishing communication between the lower pressure second flush line 32 and the flush outlet line 33. Similarly, in the event the pressure in the second input/output line 14 exceeds a certain value, that pressure will be communicated through the second flush line 32 to the second pilot line 36 which may then cause the shuttle spool 28 to be shifted upwards in the orientation of FIG. 1 thereby overcoming the bias of the spring or biasing element 37 and providing communication between the lower pressure first flush line 31 and the flush outlet line 33. The flushing circuit 11 may also include a flush flow regulator valve 38 that may also be pilot operated and used to control the flow from the flush outlet line 33 to the flush outlet 41. In short, if the pressure in the flush outlet line 33 exceeds a certain value, that pressure will be communicated through the pilot line 42 which causes the normally closed flush flow regulator valve 38 to be shifted to an open position thereby overcoming the bias of the spring or biasing element 43. The flush flow regulator valve 38 may also include a flow control device 44 that may be a fixed orifice that limits the amount of flow that could be passed from the flush outlet line 33 through the flush flow regulator valve 38. Further, because the flush flow regulator valve 38 may be pilot operated, it also acts as a low pressure relief valve that prevents flush flow to the flush outlet 41 during events that may reduce the charge pressure of the circuit 10 below a predetermined pressure value.

During certain operations of a machine or work implement in which a hydrostatic circuit 10 is incorporated, the flushing function can cause undesirable performance issues including, but not limited to jerky steering, slow steering response, loss of desired charge pressure and sluggish forward/reverse transition responses. Therefore, to avoid these problems, improved intelligently controlled flushing circuits 111, 211, 311, 411, 511, 611 are disclosed in FIGS. 2-7 which include override components that override the normal hydro-mechanical function of the flushing circuit 11. The disclosed flushing circuits 111, 211, 311, 411, 511, 611 may operate in a variety of ways including, but not limited to preventing flushing flow from exiting the flush valve 27 or the shuttle spool 28, preventing flushing flow from leaving the flushing circuit 11, preventing flow from entering the flushing circuit 11, preventing the shuttle spool 28 from shifting and/or electronically shifting the shuttle spool 28 as opposed to hydro-mechanically shifting the shuttle spool 28. Various parameters for overriding the normal hydro-mechanical function of the flushing circuits 11, 211, 311, 411, 511 and 611 are discussed below in connection with FIGS. 7-12.

Turning first to FIG. 2, a disclosed flushing circuit 111 may include a hydro-mechanically operated flush valve 27 as illustrated in FIG. 1. A flush valve override component is provided in the form of a control valve 50 disposed in the flush outlet line 33 and downstream of the flush valve 27. The control valve 50 may be a normally open solenoid control valve equipped with a biasing element 51 and a solenoid 52 that is in communication with a controller 53. As shown in FIG. 2, the control valve 50 is moveable between two positions including the open position shown in FIG. 2 and a closed position which is achieved by the controller 53 sending a signal to the solenoid 52 thereby shifting the valve 50 upward in the orientation of FIG. 2 thereby placing the check valve 54 or other blocking element in the flush outlet line 33 to prevent or block communication between the flush valve 27 and the flush outlet 41. When the valve 50 is de-energized, the flushing circuit 111 functions normally. When the controller 53 energizes the solenoid 52 thereby shifting the valve 50 to a closed position, the valve 50 prevents flushing flow from exiting the shuttle spool 28 and reaching the flush outlet 41.

Turning to FIG. 3, another flushing circuit 211 is disclosed that includes a flush valve 27 and flush flow regulator valve 38 as illustrated in FIG. 1. In the flushing circuit 211 of FIG. 3, the control valve 50 is disposed downstream of the flush flow regulator valve 38 or between the flush flow regulator valve 38 and the flush outlet 41. Thus, if a two position/two way normally open solenoid valve like that shown at 50 in FIGS. 2-3 is utilized as the flush valve override component, such a control valve 50 may be disposed upstream or downstream of the flush flow regulator valve 38 as shown in FIGS. 2-3 respectively.

Turning to FIG. 4, yet another flushing circuit 311 is disclosed that may be equipped with a flush valve 27 and flush flow regulator valve 38 as described above. Instead of the two position/two way normally open solenoid valve 50 as shown in FIGS. 2-3, the flush valve override component of the flushing circuit 311 of FIG. 4 is a two position/four way normally open solenoid control valve 60 that may be disposed upstream of the flush valve 27. The control valve 60 includes a biasing element 61 that maintains the valve 60 in a normally open position as shown in FIG. 4 and a solenoid 62 that may be linked to the controller 53. When the controller 53 sends a signal to the solenoid 62, the control valve 60 is shifted upward from the orientation of FIG. 4 to a closed position thereby blocking flow from the first and second flush lines 31, 32 to the flush valve 27. Thus, when the control valve 60 is de-energized, the flushing circuit 311 operates normally. When the controller 53 sends a signal to the solenoid 62, the control valve 60 may be shifted to a closed position thereby preventing flow from the hydrostatic circuit 10 (FIG. 1) from entering the flush valve 27.

Turning to FIG. 5, yet another flushing circuit 411 is disclosed that is similar to the flushing circuit 311 of FIG. 4. Specifically, a two position/four way normally open solenoid control valve 60 is disposed upstream of the flush valve 27, but in communication with the pilot lines 34, 36 as opposed to the flush lines 31, 32. Again, the control valve 60, when de-energized, is in the open position shown in FIG. 5. If the pressure in the first flush line 31 exceeds a certain value, that pressure will be communicated through the open control valve 60 and through the first pilot line 34 to shift the shuttle spool 28 downward thereby establishing communication between the second flush line 32 and the flush outlet line 33 as the second flush line 32 is at a lower pressure than the first flush line 31. Conversely, if the second flush line 32 is at a pressure that exceeds a certain value, that pressure will be communicated through the open control valve 60 and through the second pilot line 36 thereby shifting the shuttle spool 28 upward in the orientation of FIG. 5 and thereby establishing communication between the lower pressure first flush line 31 and the flush outlet line 33. When energized by a signal being sent from the controller 53 to the solenoid 62, the control valve 60 shifts downward in the orientation of FIG. 5 thereby blocking communication between the first and second flow lines 31, 32 and the first and second pilot lines 34, 36 respectively so that the normally closed flush valve 27 remains in its normally closed position as shown in FIG. 5. Thus, the control valve 60 of the flushing circuit 411 maintains the flush valve 27 in its normally closed position when the control valve 60 is energized.

Turning to FIG. 6, yet another flushing circuit 511 is illustrated which also acts to maintain a flush valve 127 in its normally closed position when the flushing function needs to be overridden. Specifically, the flush valve 127 of FIG. 6, as opposed to the flush valves 27 of FIGS. 2-5, is electrically or electronically activated as the flush valve 127 includes solenoids 71, 72 disposed at either end of the shuttle spool 128. The controller 53 controls the entire flushing function. That is, if pressure in the first flush line 31 exceeds a certain value, that value will be detected by the sensor 81, communicated to the controller 53, which will then send a signal to the solenoid 71 thereby shifting the shuttle spool 128 downward and providing communication between the lower pressure second flush line 32 and the flush outlet line 33. Conversely, when the pressure in the second flush line 32 exceeds a certain value, it will be detected by the sensor 82, communicated to the controller 53, which will then send a signal to the solenoid 72 thereby shifting the shuttle spool 128 upwards and establishing communication between the lower pressure first flush line 31 and the flush outlet line 33.

Finally, turning to FIG. 7, another flushing circuit 611 is illustrated which includes the pilot operated flush valve 27 and the pilot operated flush flow regulator valve 38 as described above. The flush valve override component is provided in the form of a normally open proportional flow solenoid control valve 90 downstream of the flush valve 27. It will be noted that the control valve 90 could also be disposed downstream of the flush flow regulator valve 38 as well. The controller 53 is in communication with an actuator 91 which, when fully energized, shifts the control valve 90 upward to a closed position where the check valve or other blocking element 92 blocks flow from exiting the flush valve 27. When the actuator 91 is de-energized, the control valve 90 assumes the open position shown in FIG. 7. Variable command current may be utilized to proportionally control the flushing flow rate based on system conditions. That is, instead of completely overriding the flushing function, the flushing flow rate may be reduced to an acceptable level which will not present the problems of jerky steering, slow steering response, low charge pressure and/or sluggish forward/reverse transition responses as discussed above. Thus, along with flow cancellation, the flushing circuit 611 includes the additional function of variable flushing flow.

Still referring to FIG. 7, the various disclosed methods for cancelling and activating the flushing flow function will now be described. It will also be noted that the disclosed methods for cancelling and activating the flushing flow function are also applicable to the flushing circuits 111, 211, 311, 411 and 511 of FIGS. 2-6. The controller 53 may be linked to a plurality of sensors including a pressure sensor 92 that may be coupled to the flush line 31 in FIG. 7 but could also be placed in the first input/output line 13 shown in FIG. 1. The controller 53 may also be linked to a temperature sensor 93 that is also shown coupled to the flush line 31, but could also be disposed along the first input/output line 13 (FIG. 1). Similarly, the controller 53 may be linked to a pressure sensor 94 that may be coupled to the flush line 32 as well as a temperature sensor 95 that may be coupled to the flush line 32. The pressure and temperature sensors 94, 95 could also be coupled to the second input/output line 14. The controller 53 is also linked to pressure sensors 96, 97, disposed on the other side of the hydrostatic charge pump 12 for measuring the charge pressure or pressure in the first and second input/output lines 13, 14. Thus, pressure sensors 96, 97 can also be used to measure loop pressure in a manner similar to the pressure sensors 92, 94. The controller 53 may also be linked to a temperature sensor 98 that may be coupled to the flush outlet 41 or fluid reservoir 18 for measuring the temperature of the fluid in the circuit 10 similar to the temperature sensors 93, 95. While three temperature sensors 93, 95, 98 are shown in FIG. 7, only a single temperature sensor may be needed in a closed hydrostatic circuit like the one shown at 10 in FIG. 1. Finally, the controller 53 is also linked to a steering mechanism 99 which communicates turn commands, forward and reverse commands and straight (no turn) commands to the controller 53. The importance of the variables provided by the various sensors shown in FIG. 7 will be described in connection with the flow charts of FIGS. 8-12.

Turning to FIG. 8 while still referring to FIG. 7, at part 130, the controller 53 may receive a pressure reading of fluid in the hydrostatic loop from one of the pressure sensors 92, 94, 96, 97 or another appropriately placed pressure sensor for measuring the pressure of the fluid in the circuit 10. After the controller 53 receives a pressure signal, the controller 53 determines whether the pressure is less than the predetermined pressure value at part 131. If the pressure is below a predetermined value, the flushing function is cancelled at part 132. The method illustrated in FIG. 8 may be particularly applicable when the engine speed is low thereby resulting in a low charge pressure and/or high load situations with substantial leakage of fluid. These situations can occur in certain machines such as skid steer loaders, track type loaders, various types of tractors, small wheel loaders and others. A low charge pressure can cause a work implement to pop out of a float position or a position where the implement is following the ground. Further, when the charge pressure is too low, the parking brake of certain machines may automatically be applied and, if the machine is moving, undue wear to the parking brake may occur.

Turning to FIG. 9, a method is disclosed for cancelling flushing in the event the pressure difference (ΔP) between the first and second input/output lines 13, 14 is too small. At part 133 the controller 53 receives pressure signals from two pressure sensors, such as the sensors 92, 94 or the sensors 96, 97. At part 140, the ΔP is circulated. At part 134, the controller 53 determines whether the ΔP is less than a predetermined level and, if so, the flushing is cancelled at part 135. If the ΔP is greater than the predetermined level at part 134, the method may go on to determine whether the fluid temperature is above the predetermined level at part 136. If so, flushing is actuated at part 137 to avoid over heating of the fluid.

Turning to FIG. 10, a method for cancelling flushing in the event the fluid temperature is too low is disclosed. At part 138, the controller 53 receives a temperature signal from at least one of the temperature sensors 93, 94 or 98. If the reservoir 18 and flush outlet 41 are in communication with one another, the temperature sensor 98 also reflects the temperature fluid passing through the makeup pump 19. After the temperature signal is received at part 138, the controller 53 determines whether the temperature is below a predetermined value at part 139 and, if so, flushing is cancelled at part 141. If the fluid temperature is determined to be above a predetermined value at part 139 or part 142, then flushing is activated at part 143. If the temperature of the fluid in the circuit 10 is too low, it will cause slow shifting of the flushing spool and certain machines may be subject to uncommanded motion detection or, in other words, the flushing spool moves so slowly that the machine may not be able to follow an operators command quickly enough.

Turning to FIG. 11, a method for cancelling flushing while a machine is being steered or turned is disclosed. At part 145, the controller 53 receives a signal from the steering mechanism 99 that the operator has inputted a turning command. Flushing is then cancelled at part 146 in response to the turning command. Flushing is cancelled during turning of certain machines, such as small track type tractors and skid steer loaders, because the loss of pressure and fluid caused by flushing can cause steering hesitation or a sticking or hang up of the shuttle spool 28. However, the controller 53 may include a timer and if the turning command has been applied longer than a predetermined time period at part 147, flushing may be activated at part 148 to avoid overheating the fluid. If a straight command or a return to a straight pathway command has been received at part 149, then flushing is activated at part 151.

Turning to FIG. 12, the controller 53 receives the charge pressure, most likely from either of the sensors 96, 97 at part 153. If the charge pressure is less than a predetermined minimum value at 154, then the flushing is cancelled at 155. If the charge pressure is sufficient, the controller 53 then checks to see whether it has received a turn command from the steering mechanism 99 at part 156. If so, the controller 53 determines whether the steering command has been extended past a predetermined time period at part 157. If so, flushing may be cancelled at part 158. The loop pressures and ΔP are measured and calculated at part 159 and if the ΔP is less than a predetermined minimum value at part 161, flushing may be cancelled at part 162. The controller 53 then may check the fluid temperature at part 163 and if the temperature is less than a predetermined minimum value at part 164, flushing may be cancelled at part 165. If the temperature is of a sufficient value, then the system may be flushed at part 166 in a normal fashion. While the above method were described in connection with FIG. 7, they are also applicable to the flushing circuits 111, 211, 311, 411 and 511 as shown in FIGS. 2-6.

INDUSTRIAL APPLICABILITY

As noted above, closed loop hydrostatic circuits 10 require a flushing function for purposes of maintaining clean fluid in the circuit 10, maintaining a sufficient amount of fluid in the circuit 10 in the case of leakage, controlling circuit heat by removing hot fluid and replacing it with cooler fluid, etc. However, during certain machine operational events, automated flushing circuits like that shown at 10 in FIG. 1 can be activated and cause unwanted performance issues such as jerky steering, slow steering response, low charge pressure and sluggish responses during forward and/or reverse transitions. The disclosed flushing circuits 111, 211, 311, 411, 511, 611 override the normal hydro-mechanical function of a typical flush valve 27, 127 during machine operational events when flushing is not desired but which would otherwise trigger the flushing function. The disclosed flushing circuits 111, 211, 311, 411, 511, 611 may prevent flushing flow from leaving the flush valve 27, 127 or prevent flushing flow from leaving the flushing circuit 111, 211, 311, 411, 511, 611 downstream of the flush valve 27, 127. The disclosed flushing circuits 111, 211, 311, 411, 511, 611 may also prevent flow from entering the flushing circuit 111, 211, 311, 411, 511, 611 or entering the flush valve 27, 127. The disclosed flushing circuits 111, 211, 311, 411, 511, 611 may also prevent the flush valve 27, 127 from shifting or the hydro-mechanical function of the typical flush valve 27 may be replaced by electronic control as shown by the flush valve 127 of FIG. 6. Finally, in addition to flow cancellation during certain operational events, a proportional flow solenoid control valve 90 (FIG. 7) may provide variable flushing flow control so that limited flushing can occur during machine operational events when a full flushing operation would be undesirable.

Various issues may arrive during the operation of a machine that may call for cancellation of the flushing function. For example, at slow speeds, low engine speeds, small or no load, bucking or instability may occur if the pressure difference between the first and second input/output lines 13, 14 is not sufficient. Thus, the methods of FIGS. 9 and 12 may avoid this problem. Further, during a steering operation or a turning command, insufficient pressure may be available to move the shuttle spool 28, 128, causing it to hang up. This can result in a delay in the steering of the machine (e.g., causing the machine to proceed in a straight direction as opposed to the desired turn direction). Hence, the methods of FIGS. 11 and 12 may be employed to avoid these problems. Cold fluid also causes slow shifting of the shuttle spool 28, 128 which may cause the machine to do something different than what it is being commanded to do. Thus, the methods of FIGS. 10 and 12 may avoid these problems. Further, low charge pressure can cause all sorts of problems including control pressures, and unintended application of the parking brake (not shown), thereby causing undue wear to the parking brake. Thus, the methods of FIGS. 8 and 12 may avoid these problems. 

1. A hydrostatic circuit comprising: a hydrostatic pump connected to first and second input/output lines; the first and second input/output lines connected to a hydrostatic motor to form a loop; a flush valve, a control valve and a flush outlet; the first and second input/output lines also connected to one of the flush valve or the control valve, the flush valve and control valve configured to perform three functions including providing communication between the first input/output line and the flush outlet, providing communication between the second input/output line and the flush outlet and isolating the first and second input/output lines from the flush outlet; a controller in communication with the control valve for opening the control valve and providing communication between the flush valve and the flush outlet, for closing the control valve and isolating the flush valve from the flush outlet and for reestablishing communication between one of the flush valve and the flush outlet.
 2. The hydrostatic circuit of claim 1 wherein the control valve is a normally open proportional solenoid control valve.
 3. The hydrostatic circuit of claim 1 further including a flush flow regulator valve disposed downstream of the flush valve.
 4. The hydrostatic circuit of claim 1 further including a flush flow regulator valve disposed downstream of the control valve.
 5. The hydrostatic circuit of claim 2 wherein the normally open proportional solenoid control valve is in communication with the controller and is disposed between the flush valve and the flush outlet, the normally open proportional solenoid control valve being adjustable between a fully open position providing full flow between the flush valve and the flush outlet and a fully closed position stopping flow between the flush valve and the flush outlet.
 6. The hydrostatic circuit of claim 5 further including a flush flow regulator valve disposed downstream of the normally open proportional solenoid control valve.
 7. The hydrostatic circuit of claim 6 wherein the flush flow regulator valve is pilot operated.
 8. The hydrostatic circuit of claim 1 further comprising a temperature sensor linked to the controller for communicating a temperature of fluid in the hydrostatic circuit to the controller, and if the temperature of the fluid is below the predetermined temperature, the controller closes the control valve, and if the temperature of the fluid is above the predetermined temperature, the controller opens the control valve.
 9. The hydrostatic circuit of claim 1 further including a first pressure sensor in the first input/output line and a second pressure sensor in the second input/output line, the first and second pressure sensors being linked to the controller, and if a first pressure in the first input/output line and a second pressure in the second input/output line are both below a predetermined pressure, the controller closes the control valve.
 10. The hydrostatic circuit of claim 1 further including a first pressure sensor in the first input/output line and a second pressure sensor in the second input/output line, the first and second pressure sensors being linked to the controller, the controller having a memory programmed to calculate differences between pressures sensed by the first and second pressure sensors and if said difference is less than about 20 bar, the controller closes the control valve.
 11. The hydrostatic circuit of claim 1 wherein the controller is linked to a steering mechanism, the steering mechanism for communicating an operator command for machine steering to the controller, wherein, upon receiving an operator command for machine steering from the steering mechanism, the controller closes the control valve.
 12. The hydrostatic circuit of claim 11 wherein if the steering command exceeds a predetermined time period, the controller opens the control valve.
 13. The hydrostatic circuit of claim 11 wherein if the controller receives a straight command from the steering mechanism after receiving an operator command for machine steering from the steering mechanism, the controller opens the control valve.
 14. The hydrostatic circuit of claim 1 wherein the flush outlet is connected to a fluid tank, the fluid tank including a temperature sensor that is linked to the controller, wherein if the temperature in the tank is below a predetermined temperature, the controller closes the control valve and if the temperature in the tank is above the predetermined temperature, the controller opens the control valve.
 15. A hydrostatic circuit comprising: a hydrostatic pump connected to first and second input/output lines; the first and second input/output lines connected to a hydrostatic motor to form a loop; the first and second input/output lines also connected to a flush valve, the flush valve including a spool that is moveable between a first position providing communication between the first input/output line and a flush outlet line, a second position providing communication between the second input/output line and the flush outlet line and a third position wherein the flush valve isolates the first and second input/output lines from the flush outlet line; the flush outlet line terminating at a flush outlet; a controller linked to at least one flush valve override component selected from the group consisting of a normally open solenoid control valve disposed downstream of the flush valve and upstream of the flush outlet and in communication with the controller and being moveable to a closed position for stopping flow from the flush valve to the flush outlet, a normally open solenoid control valve disposed upstream of the flush valve and in communication with the controller and being moveable to a closed position for stopping flow from the first and second input/output lines to the flush valve, a normally open solenoid control valve disposed upstream of the flush valve and in communication with the controller and being moveable to a closed position for preventing communication between the first and second input/output lines and the flush valve, and a pair of solenoids disposed at opposing ends of the flush valve and in communication with the controller for maintaining the flush valve in its normally closed position; and at least one temperature sensor linked to the controller for communicating a temperature of fluid in the hydrostatic circuit to the controller, and a plurality of pressure sensors linked to the controller for communicating pressures in the first and second input/output lines to the controller.
 16. The hydrostatic circuit of claim 15 wherein the normally open solenoid control valve disposed downstream of the flush valve and upstream of the flush outlet is a normally open proportional solenoid control valve that is adjustable between a fully open position providing full flow between the flush valve and the flush outlet and a fully closed position stopping flow between the flush valve and the flush outlet.
 17. The hydrostatic circuit of claim 15 further including a flush flow regulator valve disposed downstream of the flush valve.
 18. The hydrostatic circuit of claim 17 wherein the override component is disposed downstream of the flush flow regulator valve.
 19. A method for overriding a flushing function of a flush valve of a closed loop hydrostatic circuit, the method comprising: overriding the flushing function in response to at least one operating condition selected from the group consisting of: a) measuring a temperature of a fluid in the circuit, if the temperature of the fluid is below a predetermined minimum temperature, sending a signal to stop any flushing flow from the circuit to a flush outlet, b) measuring a loop pressure of the fluid in the circuit, if the loop pressure is below a predetermined minimum loop pressure, sending a signal to stop any flushing flow from the circuit to the flush outlet, c) measuring pressures in first and second input/output lines of the circuit, calculating a difference (ΔP) between the pressures in the first and second input/output lines, if the ΔP is below a predetermined minimum ΔP, sending a signal to stop any flushing flow from the circuit to the flush outlet, d) receiving a turning command, sending a signal to stop any flushing flow from the circuit to the flush outlet, e) receiving a turning command, sending a signal to stop any flushing flow from the circuit to the flush outlet, timing a duration of the turning command, if the duration of the turning command exceeds a predetermined maximum turning time period, sending a signal to initiate flushing flow from the circuit to the flush outlet, receiving a straight steering command, sending a signal to initiate flushing flow from the circuit to the flush outlet.
 20. The method of claim 19 wherein the receiving and sending is performed by a controller. 